THE ACTIVITY OF INTERNEURONS DURING LOCOMOTION IN THE IN-VITRO NECTURUS SPINAL-CORD

1. Less than two segments of the cervical spinal cord of the mudpuppy (Necturus maculatus) is sufficient to generate a locomotor rhythm with application of N-methyl-D-aspartic acid (NMDA). We have recorded intr acellularly from rhythmically active interneurons in these segments an d classified them according to their phase of firing within the step c ycle and their afferent input. 2. Four classes of interneurons were fo und: flexor, flexor --> extensor, extensor, and extensor --> flexor. I nterneurons that burst during the transition from flexion to extension or vice versa are referred to as ''transitional'' interneurons and re present the majority (68%) of rhythmically active interneurons studied in the mudpuppy spinal cord. 3. All flexor interneurons received only inhibitory input from cutaneous and dorsal root afferents, whereas th e flexor --> extensor interneurons that responded received only excita tory input from dorsal root and cutaneous afferents. All extensor inte rneurons and all but one extensor --> flexor interneuron received no a fferent input from the cutaneous or dorsal root afferents we stimulate d. 4. Other interneurons have been classified as ''tonic'' cells. They fire continuously when the mudpuppy is walking and are silent when th e mudpuppy is not walking. These interneurons receive no afferent inpu t from the sources tested and may be responsible for turning locomotio n on and off. 5. In conclusion, the presence of many transitional inte rneurons with specific patterns of afferent input may be required for the phasing of legged locomotion. We believe the in vitro preparation of the mudpuppy spinal cord and forelimb is an excellent model for stu dying the firing properties of interneurons during legged locomotion.